The technical advancement of optical recording started from establishment of a theory of reading out. Based on the theory, read only memory discs, i.e., CD and LD, have been developed. Thereafter, an optical recording system enabling a user to write only once, called a write once (WO) type, has been developed. The WO type recording system typically includes a pit system and a bubble system.
Lately, a magneto-optic (MO) system and a phase transition system have been proposed as an erasable recording system. For the time being, efforts are now being made to add improvements and to put them to practical use.
These optical recording techniques have their own merits and demerits as described below.
The pit type WO recording system is easy to carry out with a good choice of material and provides a high recording contrast. However, should the pits be irregular in shape, the S/N ratio would be decreased. To avoid this, it is necessary to choose a material which is capable of forming uniform and regular pits. In the case of LD, it must have an air-sandwiched structure, requiring special techniques for the production. While the pits themselves are free from deterioration, stability of the recording film itself needs due consideration. Since a single material is incapable of maintaining sufficient stability, a plurality of materials are usually used in combination in the form of a mixture or an alloy. In such a combination of materials, Te is often used as a matrix. However, even the combination of materials, whether as a mixture or an alloy, cannot get rid of the susceptibility to oxidation during long-term use and has poor durability. Besides, the process for producing the pit type recording material is complicated and is also accompanied with a variation among lots. As a matter of course, the pit type system is incapable of erasion.
The bubble type WO recording system comprises applying heat of a laser beam to the plastic material constituting a substrate to generate a gas which causes plastic deformation of an alloy, e.g., NiTi, to form bubbles. The problem of this system is poor stability of the bubbles themselves. Similarly to the pit type system, the bubble type system is incapable of erasion.
The phase transition type recording system generally utilizes phase transition between an amorphous phase and a crystal phase. As compared with the pit type system, this system is free from S/N deterioration caused by protuberance of pits, but the contrast obtained by the phase transition is not so appreciable as reached by pits and meets difficulty in assuring a high S/N ratio. Stability of the metastable amorphous phase gives rise to another problem. Stability of the constituent materials themselves, mainly comprising a chalcogenide (e.g., Te, Se), is also insufficient. Besides, some of these materials need considerable time for transition to a crystal phase, failing to exhibit satisfactory performance. Notwithstanding these disadvantages, there have been found some materials which achieve erasion utilizing amorphous/crystal phase transition, and, when compared with the MO system hereinafter described, the phase transition system has a great merit of overwriting with one beam. Therefore, this system is expected to develop as a promising erasable system.
The MO recording system is a system in which changes of angle of Kerr rotation are detected and converted to signals. Since the angle of rotation is small for obtaining a high S/N ratio, various manipulations should be added to the construction of the MO recording medium. In addition, the medium generally contains a metal susceptible to oxidation, such as Tb or Fe, which also makes the medium construction complicated, thus making the process for production difficult. Nevertheless, many manufacturers have devoted themselves studying for improving stability or S/N because of the erasability of this system.
Under these circumstances, the inventors have proposed an optical recording system utilizing phase separation as disclosed in JP-A-3-96389 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). According to this system, a laser beam of higher power is irradiated onto a recording layer to produce a quenching effect. The quenching effect induces spinodal decomposition to change reflectivity of the recording layer from low to high (low-to-high mode). On the other hand, a laser beam of lower power is irradiated to induce phase separation due to nucleation and growth of the nuclei (binordal decomposition) to change the reflectivity from high to low (high-to-low mode). The difference between the higher power and the lower power makes erasion feasible between spinodal decomposition and binordal decomposition. The first merit of this recording system is long-term stability owing to the use of polycrystalline oxides which are less susceptible to changes with time, such as a change in structure or oxidation. The second merit is a very high reaction rate. The third merit is that the oxide materials used have a high light transmission in the visible to infrared region, affording great freedom in optical and thermal designing of a multi-layered structure for optical and thermal optimization and, in addition, making it possible to increase the contrast of reproduced signals.
As stated above, a number of optical recording systems or media have so far been proposed. Of these media, CD and LD have been supplied in quantity and at low cost as read only memory (ROM) media and have been of wide prevalence owing to the strict standardization. On the other hand, while the WO type and erasable type media have also been and are being standardized, they are not so popular as CD or LD partly because of the radical technical innovations now being made in so many ways by so many makers and partly because of the expensiveness of the media themselves and the drives therefor.
These various recording systems developed to date do not always have interchangeability. Under the present situation, they are produced through the respective process and used on the respective drive. That is, interchangeability among most of different drives, discs, and production processes are extremely poor, which will cost both users and makers and give inconvenience on practical use, thus being a cause of delay of prevalence.
As an approach for eliminating the lack of interchangeability, WO type or erasable recording media having the same format as CD or LD have been studied, and some of them have been put to practical use. The CD or LD format essentially sets the reflectivity at 70% or higher. However, with the initial reflectivity being set at 70% or more, the absorbance is 30% at the highest, resulting in considerable deterioration in sensitivity.
In other words, although a WO system can be achieved by setting the initial reflectivity at 70% or more in conformity with the CD format, the sensitivity of the WO type medium cannot be set high without the cost of stability of the medium. When applied to the LD format, writing requires a greater laser power than used for CD because the linear velocity reaches at least 10 to 20 times that of CD. Accordingly, the absorbance must be set high at the cost of the reflectivity, resulting in deviation from the standard and impairment of interchangeability with an LD player. Thus, achievement of a WO system according to the CD or LD format is accompanied by difficulty in medium designing from the standpoint of reflectivity and sensitivity.